In the past, polyester polyols have been prepared by reacting the polybasic acid and polyhydric alcohol in a condensation-polymerization reaction to reach an appropriate acid value and a hydroxyl value. The synthesized polyols do not have good compatibility with polyether polyols and others that are mainly used for polyurethane forms. The polyols are not transparent at the room temperature or too high of viscosity to use at the temperature of 60° C., incurring inconvenience. Also, low stability and storage capability of the polyol has generated substantial limitations in use. Furthermore, it has many problems in mechanical property of the forms and decline in insulation, so that there has been steady demand for improvement.